Kraft waste reclamation and methodology

ABSTRACT

A method for obtaining particulate calcium carbonate having an average particle size less than about 12 microns is provided. The method includes the steps of (1) withdrawing from a pulp mill a mixture containing calcium carbonate; (2) treating the mixture to remove contaminants contained in the mixture to produce a treated mixture containing calcium carbonate and further having a chemical composition and/or purity which substantially inhibits the fusing together of calcium carbonate particulates; (3) recovering from the treated mixture particulate calcium carbonate having an average particle size less than about 12 microns. The calcium carbonate produced has a high surface area to volume ratio and is therefore highly reactive and suitable for numerous applications such as in the treatment of soil, filler paper production, paint production, and contaminant containment in coal stack emission assemblies.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. Ser. No. 10/022,086,filed Dec. 18, 2001, entitled “KRAFT WASTE RECLAMATION METHODOLOGY;”which is a continuation of U.S. Ser. No. 09/754,754, filed Jan. 4, 2001,entitled “KRAFT WASTE RECLAMATION METHODOLOGY,” now abandoned; which isa continuation of U.S. Ser. No. 09/400,243, filed Sep. 21, 1999,entitled “METHOD OF OBTAINING AND USING PARTICULATE CALCIUM CARBONATE,”now abandoned; which is a continuation of U.S. Ser. No. 08/861,333,filed May 21, 1997, entitled “METHOD OF OBTAINING AND USING PARTICULATECALCIUM CARBONATE,” now abandoned, the contents of all of which areincorporated herein in their entirety by reference.

FEDERALLY SPONSORED RESEARCH

[0002] NOT APPLICABLE

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates in general to a method forobtaining particulate calcium carbonate and methods for using same, andmore particularly, to a method for obtaining particulate calciumcarbonate having an average particle size less than about 12 micronsfrom a pulp mill and further having a chemical composition and/or puritywhich substantially inhibits the fusing together of the calciumcarbonate particulates.

[0005] 2. Background of the Art

[0006] Limestone, which is primarily calcium carbonate, has beenquarried and processed for a wide variety of uses pre-dating theconstruction of the pyramids of Ancient Egypt. The direct use oflimestone and the conversion of limestone to quicklime has continuedunabated since that time for use in the construction of buildings androads, as well as for glass formation and the purification of metals.The advent of the industrial and technical revolutions has continued theneed for high quality calcium carbonate. However, the increased need forcalcium carbonate has brought with it a demand for calcium carbonateparticle sizes below that which can be accurately measured by a standardsieve analysis. Oftentimes, the particulate size requirement is only afew microns in diameter.

[0007] The use of calcium carbonate in agricultural settings andmanufacturing applications is also well known in the art. Since WorldWar II, increasing amounts of calcium carbonate materials have beenspread on the soil of farms (hereinafter referred to as “liming”) as amethod of increasing the productivity of the soil and aligning the soilpH closer to neutral. In fact, the direct application of calciumcarbonate to soil, is the greatest single agricultural use of calciumcarbonate. Its use in agricultural applications during the 1940-1970time span accounted for approximately 70-80% of the tonnage of calciumcarbonate produced. For example, in Willamette Valley, Oreg., over150,000 tons of limestone products are used each year for soilneutralization thereby increasing the yields of a number of productsincluding grasses for seed.

[0008] Traditionally, the calcium carbonate material has been spread byself-unloading dump or tank-type trucks and the calcium carbonate hasbeen applied to the land by plowing about half of the calcium carbonateunder the soil and harrowing in the remaining half. More recently, thesoaring price of fertilizer has made the spreading of particulatecalcium carbonate an attractive and inexpensive option for farmers andother agricultural users. Indeed, it has been found that the preliminarytreatment of agricultural plots with calcium carbonate is a prerequisitein order to reap the full value from such costly fertilizers.

[0009] The use of calcium carbonate is varied across a wide spectrum ofapplications. For instance, a preponderance of crops and plants growmost profusely under neutral to slightly acidic conditions. Thus, acidicsoil in the pH range of 3.5-6.0 can be made more fertile and productivefor many crops by neutralizing soil acids. Also, the essential plantnutrients, calcium and magnesium, are supplied directly to the plants tosupport plant growth. Through liming, microbiological activities in thesoil are stimulated, thereby liberating other available plant nutrientsfrom the soil organic matter. Indirectly, liming increases organicmatter in the soil by fostering larger and more prolific growth. Greatervolumes of roots and plant residues are retained in and on the soil andthe earthworm population generally increases as the pH of the soil iselevated up to neutral.

[0010] Numerous problems have made the agricultural application ofcalcium carbonate incomplete: for instance, calcium carbonate ofsufficient size and surface area is expensive to obtain and cannot beproduced from larger sized calcium carbonate economically. Althoughliming has become a requisite in the agricultural industry, theapplication of calcium carbonate through liming has been partiallyineffective, time intensive, and potentially over applied. Furthermore,the application of powdered calcium carbonate directly to the soil isineffective, as well, because it tends to be blown away by the wind andrequires lengthy treatment times to reach the desired soil pH level.

[0011] The numerous problems of the agricultural use of calciumcarbonate are mirrored and amplified in the use of calcium carbonate forflue gas desulfurization for the reduction of acid rain, in the powerindustry. Although low sulfur coals have been utilized in order toreduce sulfur dioxide produced, thereby postponing the installation ofexpensive scrubbers, tightening environmental regulations will soonforce power plants to also use flue gas desulfurization “scrubbing”techniques. However, the size of the calcium carbonate currentlyavailable for use, is too large, thereby leading to an substantiallyineffective scrubbing process.

[0012] The dominant process for chemical pulping in the paper industryis the alkaline “Kraft” process which uses sodium hydroxide and sodiumsulfide as the primary chemical constituents. In order to make the Kraftpulping process economically feasible, the chemicals are regenerated ina series of steps, including: 1) washing the spent chemicals anddigested wood substance out of the “pulp” and collecting the resultant“weak black liquor” in large tanks; 2) evaporating the liquor inpreparation for burning in a Kraft recovery boiler which produces steamfor energy recovery and molten “smelt” for chemical recovery, whereinthe smelt drops into a tank where it is mixed with water to form “greenliquor” which contains sodium carbonate and sodium sulfide; 3) thesodium carbonate is reconverted to sodium hydroxide by using calciumoxide wherein the calcium oxide is converted into a finely dividedcalcium carbonate called “lime mud” suspended in a regenerated pulpingliquor; and 4) the calcium oxide is regenerated by burning the lime mudin a lime kiln. Before the lime mud can be burned in a cost-effectiveway, however, the lime mud must be separated from the regeneratedpulping liquor and washed. After intensive washing and denaturing steps,the lime mud contains primarily calcium carbonate with trace amounts ofcalcium hydroxide and sodium hydroxide. The calcium carbonate in thelime mud ranges in size from less than 1 to greater than 120 microns.

[0013] The regeneration of the chemicals in the Kraft pulping system,however, is not entirely effective. Typically, unreactive contaminantscome from: (1) The wood used to make the pulp; (2) Corrosion and erosionof piping and equipment; (3) sulfur compounds from the pulping liquors;and (4) sulfur gasses burned in the rotary lime kiln. A residual levelof contaminants in the lime mud results in a reburned lime havingdecreased causticizing efficiencies which translates into higher energycosts throughout the Kraft pulping process. Also, since the lime kiln isoften the bottleneck to higher pulp production rates, contaminants inthe reburned lime can decrease overall pulp production and concomitantlyincrease energy costs. If the pulp mill also has a bleach cycle, thecontaminants lower the brightness control of the pulp, therebyincreasing the bleaching costs of the pulp production system.

[0014] Thus it is an object of the present invention to provide a methodof obtaining a particulate calcium carbonate having an average particlesize less than about 12 microns from a pulp mill.

[0015] It is another object of the present invention to provide a methodof optimizing the operation of the recausticizing cycle in a pulp millthereby reducing the energy costs throughout the recausticizing cycleand maximizing pulp production.

[0016] It is a further object of the present invention to provide amethod of applying a particulate calcium carbonate having an averageparticle size less than about 12 microns to a variety of applicationswherein the size of the calcium carbonate particles is of concern.

[0017] These and other objects of the present invention will becomeapparent in light of the present Specification, claims, and Drawings.

SUMMARY OF THE INVENTION

[0018] The present invention comprises a method of obtaining particulatecalcium carbonate having an average particle size less than about 12microns. The method comprises the steps of a) withdrawing from a pulpmill a mixture containing calcium carbonate; b) treating the mixture toremove contaminants contained in the mixture to produce a treatedmixture containing calcium carbonate; and c) recovering from the treatedmixture particulate calcium carbonate having an average particle sizeless than about 12 microns.

[0019] In a preferred embodiment, the step of withdrawing from a pulpmill a mixture containing calcium carbonate further includes that themixture containing calcium carbonate may be withdrawn from either thepulp mill lime mud storage tank, the discharge of the mud filter, or thepulp mill dust control system; from all of the pulp mill lime mudstorage tank, the discharge of the mud filter, and the pulp mill dustcontrol system; or from combinations thereof.

[0020] In yet another preferred embodiment, the step of withdrawing froma pulp mill a mixture containing calcium carbonate further includes thatthe mixture containing calcium carbonate is withdrawn from the pulp millrecausticizing cycle at a constant rate so as to require fresh calciumto be added to the recausticizing cycle at a rate greater than about 25percent by weight of the requirements of the recausticizing cycle.

[0021] In another embodiment, the step of withdrawing from a pulp mill amixture containing calcium carbonate further includes that the mixturecontaining calcium carbonate is being withdrawn from the pulp millrecausticizing cycle in staggered batches so as to require fresh calciumto be added to the recausticizing cycle at a rate greater than about 25percent by weight of the requirements of the recausticizing cycle.

[0022] The present invention also contemplates a method of obtainingparticulate calcium carbonate having an average particle size about 12microns. The method comprises the steps of a) withdrawing from a pulpmill a mixture containing calcium carbonate; b) segregating theparticulate calcium carbonate from the mixture containing calciumcarbonate within the pulp mill prior to withdrawing the particulatecalcium carbonate from the pulp mill; and c) recovering from thesegregated particulate calcium carbonate, a particulate calciumcarbonate having an average particle size less than about 12 microns.

[0023] The present invention further contemplates a method of optimizingthe operation of the recausticizing cycle in a pulp mill. The methodcomprises the steps of a) withdrawing from the pulp mill recausticizingcycle a mixture containing particulate calcium carbonate; b) injectingan effective amount of a fresh calcium containing compound selected fromthe group consisting of either calcium oxide or calcium carbonate, intothe recausticizing cycle to replace the withdrawn mixture; c) treatingthe withdrawn mixture to substantially remove contaminants in themixture to produce a treated calcium carbonate mixture; and d)recovering from the treated calcium carbonate mixture a particulatecalcium carbonate having an average particle size less than about 12microns.

[0024] The present invention also contemplates a method of adjusting andenhancing the pH of soil. The method comprises the steps of a)withdrawing from a pulp mill a mixture containing calcium carbonate; b)treating the mixture to produce substantially contaminant free treatedmixture containing particulate calcium carbonate having an averageparticle size less than about 12 microns; c) admixing an effectiveamount of water to the treated mixture containing calcium carbonatehaving an average particulate size of less than about 12 microns toprovide a sprayable calcium carbonate slurry; and d) spraying aneffective amount of the sprayable calcium carbonate slurry onto the soilto penetrate the soil to a predetermined depth in order to adjust the pHof the soil. Also, if the soil is low in magnesium, a magnesiumcontaining compound, such as Mg(OH)₂, having a substantially similarparticulate size as of that of the calcium carbonate, may be added tothe slurry as well. If the soil is highly compacted, a penetrant aid mayalso be added to the slurry in order to allow the calcium carbonate topenetrate to the roots. The use of glacier water containing suspendedmicro-nutrients may also be used as a dilutant for the calcium carbonateslurry.

[0025] In a preferred embodiment of the invention, the method furthercomprises the steps of a) allowing a predetermined amount of time toelapse to permit the calcium carbonate in the sprayable slurry topenetrate the soil; and b) measuring the pH of the soil after thepredetermined amount of time.

[0026] The present invention also comprises a method for reducing acidgas contaminants from furnace and post furnace regions of power boilers,recovery boilers, and other gas streams of such constituents. The methodcomprises the steps of a) withdrawing from a pulp mill a mixturecontaining calcium carbonate; b) treating the mixture to produce asubstantially contaminant free treated mixture containing particulatecalcium carbonate having an average particle size less than about 12microns; and c) injecting an effective amount of the treated mixturecontaining calcium carbonate having an average particulate size of lessthan about 12 microns into a coal stack burning assembly.

[0027] The present invention further comprises a method for producing afiller for plastics and unbleached pulp or paper. The method comprisesthe steps of a) withdrawing from a pulp mill a mixture containingcalcium carbonate; b) treating the mixture to produce substantiallycontaminant free treated mixture containing particulate calciumcarbonate having an average particle size less than about 12 microns;and c) injecting an effective amount of the treated mixture calciumcarbonate having an average particulate size of less than about 12microns into a fiber producing assembly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0028]FIG. 1 is a diagrammatic representation of the pulping andrecausticizing cycles of a Kraft pulping mill.

[0029]FIG. 2 is a diagrammatic representation of the recausticizingcycle of a Kraft pulping mill of the present invention.

[0030]FIG. 3 is a diagrammatic representation of the recausticizingcycle of a Kraft pulping mill a second embodiment of the presentinvention.

[0031]FIG. 4 is a diagrammatic representation of the recausticizingcycle of a Kraft pulping mill a fifth embodiment of the presentinvention.

[0032]FIG. 5 is a diagrammatic representation of the recausticizingcycle of a Kraft pulping mill a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Before explaining at least one embodiment of the invention indetail, it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangements of thecomponents set forth in the following description or illustrated in thedrawings. The invention is capable of other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

[0034] The present invention provides a method for obtaining particulatecalcium carbonate having an average particle size less than about 12microns. The method comprises the steps of withdrawing from a pulp milla mixture containing calcium carbonate, treating the mixture to removecontaminants contained in the mixture to produce a treated mixturecontaining calcium carbonate, and recovering from the treated mixturecalcium carbonate having an average particle size less than about 12microns. Thereby, calcium carbonate is produced from a Kraft pulp millrecausticizing cycle. Calcium carbonate from the pulp mill is formed,classified and segregated under controlled conditions which provide forthe calcium carbonate necessary for strict industrial and agriculturalspecifications. The method of the invention produces a calcium carbonatehaving 1) reduced quantities of trace contaminants; 2) smaller overallaverage particle size; and/or 3) a mixture segregated into specific sizeranges.

[0035] Pulping and recausticizing cycles 10 of a Kraft pulp mill areshown generally in FIG. 1. A Kraft pulping process 12 generally includesthe combining of sodium hydroxide and sodium sulfide and organic woodmatter. The sodium hydroxide and sodium sulfide solubilize the lignin inthe organic wood matter thereby releasing the wood fibers which are tiedtogether in the organic wood matter by the lignin. In particular, thesodium binds with the lignin which in turn solubilizes the lignin. Afterthe lignin has been chemically removed from the organic wood matter, thereleased wood fibers are removed from the process and used in otherpaper making processes. The solubilized lignin, excess sodium hydroxideand sodium sulfide, and other trace impurities are washed out of thewood fibers and placed into a recovery boiler. In the recovery boiler,the organic matter is burned off and sodium carbonate and sodium sulfideare produced. The sodium carbonate and sodium sulfide combination isgenerally referred to as “smelt”, and is moved from the Kraft pulpingprocess 12 to a smelt dissolving tank 14 via smelt dissolving tankconduit 15.

[0036] In the smelt dissolving tank 14, the smelt is mixed with water toform a slurry commonly referred to as a “Green Liquor.” The Green Liquoris then moved from the smelt dissolving tank 14 to a Green Liquorclarifier 16 via Green Liquor clarifier conduit 19. The Green Liquorcontains, in addition to the sodium carbonate and sodium sulfide, traceamounts of solids such as burned dark carbon and metallics. The GreenLiquor clarifier 16 separates the liquid (which contains the sodiumcarbonate and sodium sulfide) from the solids. The Green Liquorclarifier 16 settles the solids out of solution and an outlet 17 allowsfor the controlled removal of the solids in such a manner as to minimizethe removal of the liquid. In such a manner, the Green Liquor issubstantially cleaned of solid contaminants.

[0037] The clean Green Liquor thereafter leaves the Green Liquorclarifier 16 and enters a slaker 18. The slaker 18 introduces an amountof calcium oxide to the Green Liquor (which contains sodium carbonateand sodium sulfide). The calcium oxide therein reacts with the sodiumcarbonate to produce sodium hydroxide and calcium carbonate, while thesodium sulfide remains in solution and does not react. The calciumcarbonate precipitates out of solution and the solution which containsthe sodium hydroxide and sodium sulfide is commonly referred to as a“White Liquor.” After the reaction has occurred, the precipitatedcalcium carbonate and the White Liquor are moved to a White Liquorclarifier 20.

[0038] The White Liquor clarifier 20 separates the White Liquor solutionfrom the precipitated calcium carbonate. The White Liquor solution isconveyed back to the Kraft pulping process 12 via the White Liquorconduit 21. The White Liquor, containing sodium hydroxide and sodiumsulfide, is therein reused to separate the wood fibers from the ligninin the Kraft pulping process 12, as described hereinabove.

[0039] The precipitated calcium carbonate (also referred to as “limemud”) in White Liquor clarifier 20 is removed from White Liquorclarifier 20 and conveyed to mud washer 24 via lime mud conduit 22. Thelime mud which exits the White Liquor clarifier 20 is approximately 50%calcium carbonate suspended in a residual amount of White Liquor. Mudwasher 24 adds water to the lime mud and White Liquor suspension inorder to dilute the lime mud further. After the water has been added tothe lime mud, the lime mud is allowed to settle out of the solution. Thelime mud is then removed from the mud washer 24 and conveyed to the mudstorage 26 via mud storage conduit 28. The remaining diluted WhiteLiquor in mud washer 24 is pumped into the Kraft pulping process 12 byWhite Liquor pumping apparatus 30.

[0040] The lime mud entering the mud storage 26 is approximately 25-35%solids. Further, the calcium carbonate in the lime mud ranges in sizefrom less than 1 micron to greater than 120 microns. The lime mud isthereafter conveyed from mud storage 26 to a mud filter 32 via a mudfilter conduit 34. The mud filter 32 washes the lime mud with water andseparates the lime mud from the resulting solution. After the washing,the lime mud is removed from the mud filter 32 and transported to arotary lime kiln 36 via rotary lime kiln conduit 38. The lime mudleaving the mud filter 32 and entering rotary lime kiln 36 isapproximately 70-80% solids.

[0041] The rotary lime kiln 36 converts the calcium carbonate in thelime mud to calcium oxide through a calcination process. After thecalcination process has finished, the converted calcium oxide istransported to a storage silo 40 by means of a silo conduit 42. Thestorage silo 40 holds the converted calcium oxide until it is needed bythe slaker 18 in the conversion of sodium carbonate and calcium oxide tosodium hydroxide and calcium carbonate. In this manner, the sodiumhydroxide and sodium sulfide used in the Kraft pulping process 12 arerepeatedly regenerated for subsequent use.

[0042] The pulping and recausticizing cycles of the Kraft pulp mill,described above, oftentimes accumulate unreactive contaminants, suchas 1) contaminants from the particular wood that is used for pulping, 2)unburned contaminants from the Kraft pulping process, 3) iron compoundsfrom mill conduit piping, and 4) various other impurities found in thechemicals introduced into the process in order to make up the chemicallosses which occur during processing. Furthermore, unreactive calciumsulfate may be formed from sulfur dioxide which reacts with the calciumoxide within the rotary lime kiln 36. Contaminant build-up results in 10to 20% more calcium oxide (reburned lime) being needed in the slaker 18for each conversion reaction of sodium carbonate to sodium hydroxide andcalcium carbonate. The contaminants may also fuse the small calciumcarbonate particles produced in the conversion reaction of sodiumcarbonate to sodium hydroxide into larger calcium carbonate agglomerateswhich tend to make them less desirable and/or less reactive.

[0043] The present invention, as shown in FIG. 2, contemplates theremoval of the precipitated calcium carbonate from the recausticizingcycle 23, treating the recovered precipitated calcium carbonate toremove any residual contaminants remaining therein, and recovering fromthe treated mixture a particulate calcium carbonate having an averageparticle size less than about 12 microns. As described above, theprecipitated calcium carbonate enters White Liquor clarifier 20 whichthereby separates the White Liquor solution from the precipitatedcalcium carbonate. The precipitated calcium carbonate is then conveyedto mud washer 24 via lime mud conduit 22. After being washed, the limemud is conveyed to mud storage 26 via mud storage conduit 28. Theprecipitated calcium carbonate and any residual White Liquor solution isthereafter removed from mud storage 26 to a storage assembly 46 via astorage assembly conduit 44. It is also contemplated that any excesssoluble hydroxide can be treated by either (1) oxidizing the mud in themud storage in order to stabilize the sulfides and/or (2) usingcarbonated water to wash the mud on the mud filter.

[0044] It is also contemplated, and as shown in FIG. 3, that theuntreated calcium carbonate solution be withdrawn from mud filter 32 toa storage assembly 46 a via a storage assembly conduit 44 a.Furthermore, the calcium carbonate, as shown in FIG. 4, may be removedfrom the rotary lime kiln 36 to a storage assembly 46 b via a storageassembly conduit 44 b, or from rotary lime kiln dust screens (not shown)or by any combination thereof. By choosing the withdrawal site of theuntreated calcium carbonate solution, the calcium carbonate particlesize distributions can be more exactingly controlled. It is alsocontemplated that calcium carbonate particles having an average sizeless than about 12 microns be removed from the recausticizing cycle 23,as shown in FIG. 5, by pumping the solution from the mud storage 26through a hydrocyclone 48 or other classification device. The materialhaving an average size greater than about 12 microns is returned to themud storage 26 via a return conduit 50. The material having an averagesize less than about 12 microns is conveyed to the storage assembly 46via conduit 52 for further treatment and/or segregation.

[0045] The calcium carbonate taken off mud filter 32, which is typicallyat about 70-80% solids, may be dried with a small rotary dryer or otherdrying device (not shown) in order to provide a calcium carbonate whichmay be more economically and efficiently shipped. For certainapplications, sufficient dryness could be obtained by adding additionalcalcium oxide or calcium carbonate to the removed and treated calciumcarbonate, followed by drying the combination with a rotary dryer. Itwould be apparent to one of ordinary skill in the art that any methodwhich can efficiently dry the removed calcium carbonate is contemplatedfor use.

[0046] Storage assembly 46 is a storage facility capable of treating thecalcium carbonate solution in order to further remove any residualcontaminants which may be in the solution. The calcium carbonatetreatment may consist of single or multiple washings with water or anyother suitable chemical treatment. It is also contemplated that thecalcium carbonate treatment involve a chemical treatment, whereby anyimpurities or contaminants in the mixture are solubilized and removedand/or the impurities or contaminants may be converted to some harmlessor beneficial by-product. It is also contemplated that two or morediluting and/or rethickening steps may be required in order to removethe impurities or contaminants. Also, if the calcium carbonate solutioncontains excess sodium, the storage assembly 46 may also contain aconductivity meter capable of measuring the amount of sodiumcontaminants in the solution and maximize the purification treatment ofthe calcium carbonate solution to remove the contaminant sodium. Inparticular, the conductivity of the calcium carbonate solution should beless than 1800 micromhos. In any event, one of ordinary skill in the artwould understand that any method of treating the precipitated calciumcarbonate solution, so as to substantially remove impurities ofcontaminants remaining therein, is contemplated for use.

[0047] The storage assembly 46 is also capable of recovering, from theabove-described treated calcium carbonate, a particulate calciumcarbonate having an average particle size less than about 12 microns. Inone embodiment, the storage assembly 46 may contain a hydrocyclone, orother such classification device which would be known to one of ordinaryskill in the art. The hydrocyclone, or other segregation device, iscapable of removing a particular range of particle sizes based upon theunit design and the resultant pressure drop across the unit. In terms ofoperational control, a higher pressure drop shifts the particle sizesplit so that the discharge from the top of the cyclone contains lesstotal mass flow of the particulate calcium carbonate, but will containparticulate calcium carbonate having a smaller particle size. It is alsocontemplated that the recovery of particulate calcium carbonate havingan average size less than about 12 microns may involve more than onesegregation step. For instance, two or more passes through thehydrocyclone or other segregation device may be required in order toobtain a calcium carbonate particulate having an average size less than12 microns.

[0048] Alternate methods may be employed within the storage assembly 46in order to segregate out a particulate calcium carbonate having anaverage particle size less than 12 microns. A settling chamber orclarifier may be included with the storage assembly 46 which allows thelarger particles to settle out of solution first, thereby allowing forthe preferential selection of smaller particles. Also, a liquid grindingor impacting assembly may be included in the storage assembly 46 inorder to break up larger sized particles that may escape removal by thehydrocyclone or other segregating device. Of course, any of thesesegregating devices may be used singularly or in combination with any ofthe others and may also comprise an additional assembly separate from,yet connected to, storage assembly 46. In any event, one of ordinaryskill in the art will understand that any method of segregating acalcium carbonate particulate having an average particle size less than12 microns out of solution, is contemplated for use.

[0049] Pulp mills traditionally operate the recausticizing cycle so thatup to 1.5% sodium, expressed as Na₂O, is in the process since thisamount of sodium helps to reduce dust and encourages the formation of apellet sized calcium oxide product. This minute level of sodium,however, causes the calcium carbonate to form agglomerates of anincreased size, as described hereinabove. In this regard, the effect ofthe sodium on the calcium carbonate appears to be cumulative. In orderto minimize the number of calcium carbonate agglomerates formed, thecalcium carbonate is preferentially removed from the mud storage 26before it has been through rotary lime kiln 36.

[0050] Currently, pulp mills add approximately 5% calcium oxide to therecausticizing cycle (hereinafter referred to as “new lime”) in order topurge an amount of the contaminants formed during each cycle. Even withthis purge in the recausticizing cycle, however, the amount ofcontaminants in the system is such that the contamination levels in thecycle are increased by a factor of twenty. The effectiveness of anycontaminant purge is expressed by the “percentage available calciumoxide” found in the material leaving rotary lime kiln 36. For example,calcium oxide produced in the rotary lime kiln 36 often has less thaneighty-five percent (85%) calcium oxide available, whereas new lime hasgreater than ninety-two percent (92%) calcium oxide available.Therefore, the more calcium oxide produced in the rotary lime kiln 36that is used for each ton of pulp produced, causes several dead loadcosts. Furthermore, if the rotary lime kiln 36 is the bottleneck in therecausticizing cycle 23, there is a correlative loss in pulp productionin the sodium cycle 13.

[0051] It is contemplated that the removal of the calcium carbonate fromthe recausticizing cycle 23 be accomplished in such a manner so that newcalcium, such as calcium oxide, be added to the recausticizing cycle 23at a rate greater than about 25 percent by weight of the requirements ofthe recausticizing cycle 23. The mixture containing the calciumcarbonate may be withdrawn from the recausticizing cycle 23 in either acontinuous stream or in staged increments, so long as the 25 percent byweight withdrawal requirement is met.

[0052] The present invention also contemplates the optimization of aKraft pulp mill. The method of optimizing the Kraft pulp mill includesthe steps of: (A) withdrawing from the pulp mill recausticizing cycle amixture containing particulate calcium carbonate; (B) injecting aneffective amount of a fresh calcium containing compound selected fromthe group consisting of calcium oxide and calcium carbonate, into therecausticizing cycle to replace the withdrawn mixture; (C) treating thewithdrawn mixture to substantially remove contaminants in the mixture toproduce a treated calcium carbonate mixture; and (D) recovering from thetreated calcium carbonate mixture a particulate calcium carbonate havingan average particle size less than about 12 microns.

[0053] It is contemplated that the calcium carbonate be withdrawn at arate greater than 5% and that the new lime be introduced into storagesilo 40 thereby substantially purging the recausticizing cycle 23 ofcontaminants, and eliminating the rotary lime kiln 36 as a pulp millpulp production bottleneck. Traditionally, the limiting factor on thedegree of the purge was based on economic considerations since thepurged “lime mud” or calcium carbonate mixture had to be hauled away orstored in some manner. If the rotary lime kiln 36 is the bottleneck tohigher pulp production rates, reducing the amount of calcium oxideneeded to be produced by the rotary lime kiln 36 results in the calciumoxide being added to the slaker having a higher percent availablecalcium oxide content which thereby translates into increased pulpproduction capacity without a significant capital expenditure. Theoptimization of the recausticizing cycle 23 of the pulp mill translatesinto lower overall energy costs within Kraft pulping process 12. It isalso contemplated that any residual moisture in the calcium carbonatemay be reduced and/or eliminated by passing it through an additionalrotary dryer or by using the existing rotary lime kiln as a dryer ifproduction requirements are not affected.

[0054] If calcium carbonate is removed from the rotary lime kiln 36 ordust screens, calcium carbonate dust levels within the rotary lime kiln36 will be minimized. Kraft pulp mills that utilize bleach plants willbenefit from better brightness control and lower bleaching costs due tothe decrease in pulp mill contaminants that are removed with the calciumcarbonate and the increased reactivity of the calcium oxide that isintroduced into slaker 18.

[0055] Furthermore, the rotary lime kiln 36 is the back end of thepulping process and there is a general lack of knowledge in the field asto the recausticizing cycle. For example, pulp mills generally do nothave a different set of specifications and procedures for the additionof fresh calcium oxide. In fact, operators often attribute poor settlingof the calcium oxide mud with the addition of fresh calcium oxide ingeneral, rather than the fact that there may be too high a concentrationof calcium oxide in the recausticizing cycle overall. It is contemplatedthat fresh calcium oxide, therefore, be added at a rate of 10-20% lessthan the amount of calcium oxide produced by the rotary lime kiln 36.The calcium oxide mud produced by the addition of 10-20% less freshcalcium oxide versus calcium oxide produced in the rotary lime kiln 36settles very quickly even though the particle size is much smaller.

[0056] It is contemplated that the calcium carbonate produced by thepresent invention will also benefit users of calcium carbonate productsby lowering the capital costs inherent in traditional methods ofprocessing calcium carbonate, such as manual grinding or crushingsystems. Rather than expensive rock handling and grinding equipment, theoptimization of pulp mill operations and the resultant production ofcalcium carbonate requires minimal (1) wet or dry product storage; (2)transfer and transport equipment; and (3) classification or segregationequipment. The present invention, therefore, contemplates a method ofoptimizing a pulp mill recausticizing cycle and in return producing aparticulate calcium carbonate product which is highly reactive andeconomically produced.

[0057] The present invention also contemplates a method for adjustingand enhancing the pH of the soil. The method generally entails the useof the calcium carbonate particles removed from a Kraft pulp mill. Inparticular, the method comprises the steps of (A) withdrawing from apulp mill a mixture containing calcium carbonate; (B) treating themixture to produce a substantially contaminant free treated mixturecontaining particulate calcium carbonate having an average particle sizeless than about 12 microns; (C) admixing an effective amount of water tothe treated mixture containing calcium carbonate having an averageparticulate size of less than about 12 microns in order to provide asprayable calcium carbonate slurry; and (D) spraying an effective amountof the sprayable calcium carbonate slurry onto the soil in order topenetrate the soil to a predetermined depth in order to adjust the pH ofthe soil. The method may also comprise the additional steps of (1)allowing a predetermined amount of time to elapse in order to permit thecalcium carbonate in the sprayable slurry to penetrate the soil; and (2)measuring the pH of the soil after the predetermined amount of time. Inthis manner the soil may be treated so as to neutralize its acidicnature through a method which is quick, efficient, and easilyadministered.

[0058] Particle size for agricultural calcium carbonate traditionallyaverages 84 microns or more. With an average particle size of less than12 microns, the full benefit of the calcium carbonate treatment will berecognized in the same growing season as its application. The smallparticle size of the calcium carbonate removed from the pulp mills,which is less than about 12 microns, is well below the particle size ofground calcium carbonate which is currently produced and used by theagriculture industry. In fact, attempting to grind limestone to aparticle size of less than about 12 microns would be economicallyinfeasible. Furthermore, the relatively small size of the removedcalcium carbonate results in a product having a high volume to surfacearea ratio. Thus, the removed and treated calcium carbonate is highlyreactive and results in a relatively inexpensive and effective soiltreatment.

[0059] It is also contemplated that the removed and treated calciumcarbonate can further be improved by grinding the particles into adesired size range even less than 12 microns through the use of atouch-up grinder. For instance, the power industry often uses calciumcarbonate to scrub sulfur dioxide from their flue gases which areexhausted into the atmosphere. In order to maximize the calciumcarbonate's reaction and application efficiency, it is ground to itssmallest economic particle size.

[0060] The present invention also contemplates a method for reducingacid gas contaminants from furnace and post furnace regions of powerboilers, recovery boilers, and other gas streams of such constituents.The method comprises the steps of (1) withdrawing from a pulp mill amixture containing calcium carbonate; (2) treating the mixture toproduce a substantially contaminant free treated mixture containingparticulate calcium carbonate having an average particle size less thanabout 12 microns; and (3) injecting an effective amount of the treatedmixture containing calcium carbonate having an average particulate sizeof less than about 12 microns into a coal stack burning assembly. Thecalcium carbonate injected into the coal stack burning assembly ishighly reactive with the acid gases, such as SO₂, HCl, because of itshigh surface area to volume ratio. The calcium carbonate reacts with theacid gases to produce calcium sulfite, calcium sulfate and calciumchloride and H₂O, thereby lowering the amount of acid gases emitted fromthe coal stack burning assembly. In such a manner, the amount of acidgas contaminants is substantially reduced.

[0061] The present invention further contemplates a method for producinga filler for unbleached pulp or paper. The method comprises the steps of(1) withdrawing from a pulp mill a mixture containing calcium carbonate;(2) treating the mixture to produce a substantially contaminant freetreated mixture containing particulate calcium carbonate having anaverage particle size less than about 12 microns; and (3) injecting aneffective amount of the treated mixture calcium carbonate having anaverage particulate size of less than about 12 microns into a fiberproducing assembly.

[0062] Because of its lower brightness, the calcium carbonate producedby the invention would have to be bleached in order that it be used inthe production of paper from pulp. The brightness of the particulatecalcium carbonate can also be enhanced by the addition of bleachingagents at the lime mud filter. However, it may be used in unbleachedpaper and paper board, and its use in such a product would cut pulpcosts. However, even if the produced calcium carbonate is too dark toput into unbleached linerboard or sack paper, it may be treated with ableaching agent and thereby improve its brightness.

[0063] The calcium carbonate produced by the invention could also bereadily used as a substitute for ground calcium carbonate which is usedin the manufacture of some types of paints which do not require a normalbrightness factor, such as for use in dark colors of paint. It is alsocontemplated that the calcium carbonate could be bleached to improve thefinal brightness of the paint. As with paint, the addition of thecalcium carbonate to plastics is also contemplated after bringing theproduct to dryness.

[0064] Thus, it should be apparent that there has been provided inaccordance with the present invention a method for obtaining particulatecalcium carbonate having an average particle size less than about 12microns that fully satisfies the objectives and advantages set forthabove. Although the invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

What I claim is:
 1. A method for obtaining particulate calcium carbonate product having an average particle size less than about 12 microns, comprising the steps of: withdrawing from a kraft pulp mill running a kraft pulp mill process an alkaline mixture containing calcium carbonate wherein the alkaline mixture containing calcium carbonate is withdrawn during a recausticizing cycle of the kraft pulp mill process; treating the alkaline mixture containing calcium carbonate so as to remove contaminants contained in the alkaline mixture to thereby produce a treated alkaline mixture substantially free of contaminants and containing calcium carbonate; and recovering from the treated alkaline mixture a particulate calcium carbonate product having an average particle size less than about 12 microns, wherein the treated alkaline mixture has been treated such that the particulate calcium carbonate product has a substantially neutral pH.
 2. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is withdrawn from a pulp mill lime mud storage tank.
 3. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is withdrawn from a discharge of the mud filter.
 4. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is withdrawn from a pulp mill dust control system.
 5. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is withdrawn from a plurality of a pulp mill lime mud storage tanks, a mud filter, a pulp mill dust control system, and combinations thereof.
 6. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is withdrawn from a pulp mill recausticizing cycle at a constant rate so as to require fresh calcium to be added to the recausticizing cycle at a rate greater than about 25 percent by weight of the requirements of the recausticizing cycle.
 7. The method of claim 1, wherein in the step of withdrawing from a pulp mill a mixture containing calcium carbonate, the mixture containing calcium carbonate is being withdrawn from a pulp mill recausticizing cycle in staggered batches so as to require fresh calcium to be added to the recausticizing cycle at a rate greater than about 25 percent by weight of the requirements of the recausticizing cycle. 